The Origin of Penumbral Structure in Sunspots: Downward Pumping of Magnetic Flux

This paper offers the first coherent picture of the interactions between convection and magnetic fields that lead to the formation of the complicated filamentary structure of a sunspot penumbra. Recent observations have revealed the intricate interlocking-comb structure of the penumbral magnetic field. Some field lines, with associated Evershed outflows, plunge below the solar surface near the edge of the spot. We claim that these field lines are pumped downward by small-scale granular convection outside the sunspot. This mechanism is demonstrated in numerical experiments. Magnetic pumping is a key new ingredient that links several theoretical ideas about penumbral structure and dynamics; it explains not only the abrupt appearance of a penumbra as a pore increases in size but also the behavior of moving magnetic features outside a spot. Subject headings: MHD — Sun: magnetic fields — Sun: photosphere — sunspots On-line material: color figures